1. Field of the Invention
The present invention relates to a liquid crystal display device and a fabrication method thereof, and particularly, to a liquid crystal display device having both GOLDD (Gate Overlapped Lightly Doped Drain) type and LDD (Lightly Doped Drain) type thin film transistors and a fabrication method thereof.
2. Description of the Related Art
Recently, research for lighter, more compact flat panel display devices have resulted in liquid crystal display (LCD) devices being mass-produced and widely used. LCD devices usually use thin film transistors (TFT). TFTs in a LCD device are used as switching devices to individually drive each respective pixel. A TFT includes a semiconductor layer having a channel formed therein through which a current flows, a gate electrode for controlling the currently by applying a scanning signal to turn the current on and off a source electrode for inputting data signals, and a drain electrode for outputting data signals.
The LCD includes a pixel region having a plurality of pixels therein for displaying real images, and a driving circuit unit for applying various signals to the pixel region. The TFT makes up all of the pixel region and the driving circuit unit. For Chip On Glass (COG) type LCD, which has the driving circuit unit and pixel region on the same substrate, research to form a compact LCD has been mainly achieved by providing the driving circuit unit with a polycrystalline TFT.
The TFT formed in the driving circuit unit has to have higher electric mobility as compared with the TFT formed in the pixel region. Accordingly, the polycrystalline TFT having high electric mobility is usually applied in the driving circuit unit.
Recently, attempts have been made to further minimize the LCD to form a lighter and more compact display apparatus. However, since the pixel region in which images are displayed cannot be further reduced, (the size of this pixel region has already been established), the size reduction of the LCD is substantially achieved by reducing the area of the driving circuit unit. Thus, when the area of the driving circuit unit is reduced, the size of the TFT arranged in the driving circuit unit is inevitably reduced as well. The reduction of the TFT is achieved by reducing the length of the channel. Reducing the length of the channel, however, make the channel layer prone to damage by generating hot carriers on the channel. The hot carriers trapped within the channel change the threshold voltage of a device creating a defect.
To solve this problem, a LDD (Lightly Doped drain) type TFT has been introduced. The LDD type TFT has a low concentration impurity region. The LDD region is formed adjacent to the channel layer and a high concentration impurity region is formed outside of the LDD region. Furthermore, since LDD type TFT hardly generates any off current, it may prevent leakage currents that cause degradation of image quality.
However, in the LDD type TFT, there is a limit on how much the channel length can be reduced. Reliability of the TFT declines as the channel length gets shorter. Therefore, when the TFT having short channel length is applied to display apparatuses of high image qualities such as HDTV, the channel may be damaged due to hot carrier effects.
A TFT having a GOLDD (Gate Overlapped LDD) type is proposed to solve those problems. In the GOLDD type TFT, since a gate electrode is overlapped with an LDD region, it is possible to form a short channel. As a result, a small-sized TFT with reliability can be fabricated.
Hereinafter, a fabrication process for the GOLDD type TFT according to the related art is discussed with reference to FIGS. 1A through 1F.
First, as shown in FIG. 1A, a buffer layer 102 is formed on a substrate 101, which is made of a transparent material like glass. Next, a semiconductor layer 103 is formed on the buffer layer 102 by depositing an amorphous semiconductor such as a silicon and patterning it. Thereafter, a photoresist pattern 104 is formed on the semiconductor layer 103 and then low concentration impurity ion (i.e., n− ion) is injected into the exposed region of the semiconductor layer 103 not blocked by the photoresist pattern. The result is the formation of a channel layer 103a and low concentration impurity region 103b, the n− region.
Next, as shown in FIG. 1B, after removing the photoresist pattern 104 on the channel layer 103a, a laser beam is irradiated on the semiconductor layer 103 to crystallize the amorphous semiconductor layer. When the laser beam is irradiated on the semiconductor layer 103, the impurity ion injected in the n− region 103b is activated.
Next, as shown in FIG. 1C, a gate insulating layer 105 is formed on the semiconductor layer 103. Thereafter, a metal layer 106 is formed on the gate insulating layer 105.
Referring to FIG. 1D, a photoresist pattern 107 is formed on the metal layer 106 and photolithography process is performed to create gate electrode 106a. The size of the gate electrode 106a is greater than the channel layer 103a. 
Next, referring to FIG. 1E, high concentration impurity ion (n+ ion) is injected into the n− region 103b by using the gate electrode 106a as a mask. A part of each n− layer 103b (i.e., regions where are not blocked by the gate electrode 106a) becomes high concentration impurity region 103c (n+ region). As a result, the gate electrode 106a is overlapped with low concentration impurity regions 103b′. 
After forming the high concentration impurity layer 103c, as shown in FIG. 1F, a passivation layer 108 is formed on the gate electrode 106a. A conductive layer is deposited on the passivation layer 108 and is etched to form a source electrode 109 and a drain electrode 110.
The source electrode 109 and the drain electrode 110 are connected to the n+ region 103c respectively through contact holes exposing the n+ region 103c. 
Accordingly, the GOLDD type TFT has low concentration impurity ion region and high concentration impurity ion region similar to that of LDD type TFT. The GOLDD type TFT has superior reliability but cannot readily be reduced in size. On the other hand, the LDD type TFT is advantageous for its small-size but its reliability is degraded due to minimization.